The invention relates to methods for automatic focusing a charged particle beam. Furthermore, this invention relates to autofocus of a charged particle beam under conditions of charge on the examined specimen.
Due to their high resolving power, beams of negatively or positively charged particles are used for the examination of specimen. Compared to optical light, the resolving power of a beam of charged particles is several magnitudes higher and allows for the examination of much finer details. Accordingly, charged particle beams, especially electron beams, are used for the inspection of masks and wafers used in semiconductor technology, which requires a very high resolution.
In order to get a sharp and clear picture of the specimen, it is necessary to focus the charged particle beam on the specimen. Due to the fact that most specimen show some variation in their surface height, the charged particle beam has to be refocused from time to time so that a certain image quality is maintained. The focusing of the charged particle beam is usually done by either varying the current supplied to the objective lens, and thereby changing the focal length, or changing the working distance to the specimen using a Z-stage.
Performing autofocus using changes in the current supplied to the objective lens is somewhat problematic. Due to the high inductance of the magnetic lens coils, the response is very slow and rapid changes in the focal point cannot be made. Furthermore, a variation in the objective lens current causes a change in the magnification and a rotation of the scanning direction of the charged particle beam, changes which are difficult to characterize exactly for each working situation. Accordingly, there is need for an improved method for autofocus of charged particle beam.
A different method to focus an electron beam is described in U.S. Pat. Nos. 4,999,496, 4,999,496 recognizes that changes in focal length or working distance also cause changes in the magnification. In order to compensate for changes in magnification, it teaches to change the beam voltage until a focus is achieved, and use the amount of change to compensate for the changes in magnification.
Apart from surface topography, it has been found that the presence of an electric potential on the surface of the specimen can also lead to a serious degrade in the image quality and focus. The electric potential on the surface of the specimen may be caused by an unintentional or unavoidable charging of the specimen. The electric potential on the surface of the specimen may also be caused by intentionally applying a voltage to the specimen. A voltage can be applied to a wafer, for example, in order to obtain voltage contrast imaging which is used to detect shorts in a circuit. These effects are yet to be countered effectively.
Some specimen, like for example semiconductor wafers, contain a plurality of different target areas that have to be examined. Due to the fact that most specimen are warped and that they usually exhibit some undulations in their surface height, it is often necessary to refocus the charged particle beam on each target area. Unfortunately, this refocusing of the charged particle beam on each target area is a rather time consuming operation. One commonly used way is to focus an optical system on each target area and to learn each target area height. Once each target area height is known, the electron beam can be driven to the right focus on a specific target area if the function of electron beam focus versus the target area height is known. However, apart from being a rather time consuming operation, the process described above leads to additional disadvantages. The transparency of the target area may lead to significant errors in the focusing of the electron beam because the optical system may measure the target area height incorrectly. Furthermore, an electric potential being present in the target area will usually lead to a serious degradation of the image quality. Accordingly, there is also a need for a faster and more reliable method for focusing a beam, especially a charged particle beam, onto a target area.
Prior art methods for focusing a charged particle beam often require a definition of a focus search range for the focus setting and than form corresponding image signals in different focus states of the beam in a way that the focus search range is covered. Focus scores are computed for every image and the scores of all the images are then compared. The value of the parameter corresponding to the image having the highest score is selected for the actual measurement. In many cases, however, the predetermined search range does not include the best focus state and therefore the prior art approach is not capable of finding the best focus state. Furthermore, even if the predetermined search range does include the best focus state, a lot of time is usually wasted, because image signals have to be formed throughout the whole search range. Accordingly, there is also a need for a faster method for focusing a charged particle beam, that does not require a predetermined search range.
The present invention provides a method for automatic focusing a charged particle beam, which compensates for both surface topography and electric charge on the specimen. According to one embodiment of the invention, xe2x80x9cglobalxe2x80x9d focusing is done using a Z-stage, while correction, especially for charge, are made by changing the beam energy. Thus, a fast response is provided for charging effects, while avoiding changes in magnification or scanning direction.
According to one feature of the invention, the negative effects of an electric potential being present on the surface of the specimen are overcome by analyzing scores of images achieved with different beam energies and by adjusting the beam energy according to the analysis. Thereby, image quality problems caused, for example, by a wafer charged with a static charge and showing a surface potential in the range of a couple of hundreds volts, for example +100 to xe2x88x92400 V, can be corrected without any image rotation. Furthermore, there is no necessity to correct stigmation or other beam alignments. Moreover, according to the inventive solution, the magnification remains constant. Accordingly, there is no need to provide a magnification compensation. Compared to an adjustment of the current through the objective lens, the response speed is considerably increased and any hysteresis phenomena, usually connected with a change in the objective lens current, can be avoided. The present invention also provides an apparatus for the examination of specimen that is capable of performing this improved method.
According to a further aspect, the present invention provides a method for mapping a charge distribution on the surface of a specimen. The method uses the information contained in the corrections, that are made by changing the beam energy, in order to compensate for electric charge on the specimen. For each scanned location the beam energy or the changes to the beam energy together with the coordinates of the scanned location are recorded, so that a map showing the charge distribution and/or the field distribution on the surface of the specimen can be constructed. Such a map can be of very high value, for example, when a failure in a circuit design has to be found. The present invention also provides an apparatus for providing a map of the charge distribution on the surface of a specimen.
According to a still further aspect, the present invention provides an improved method for focusing a beam onto a target area of a specimen. The method uses images of a repeating pattern that lies in the path of the beam on its way to the actual target area. Scores of these images that are achieved with different focus settings are analyzed and the correct focus setting is then used once the beam has reached the target area. This method has the advantage that the information about the correct focus is gathered during the time period in which the specimen and/or the beam is moved to the actual target location. Furthermore, this is done through an analysis of the information obtained by the images created by the beam itself. The beam is basically in focus when it reaches the target area. Accordingly, a lot of time, that usually has to be used for collecting all the information about the surface height of each target area, can be saved and the throughput is increased. The present invention also provides an apparatus for the examination of specimen that is capable of performing this improved focusing method.
According to a further aspect, the present invention provides a further improved method for automatic focusing a charged particle beam. The method uses the image scores of two successive focus states and changes the focus state then in a direction that will yield an improvement in the scores. This is repeated until the score has reached a predetermined value or the changes in the scores (or in the focus states) are smaller than a predetermined value that is, for example, smaller or equal to the depth of focus of the beam. The method has the advantage that there is no need to define the focus search range in advance and the method in most cases converges much faster than the methods used in the prior art.